The invention pertains to large, stationary, rotary cranes of the type which are assembled on the job site and which can be disassembled after use and moved to a different location. These rotary cranes have a boom and a mast for swinging between a generally horizontal position from which they are erected to an upwardly extending, working position. A mobile, self-propelled power unit is provided for furnishing the power for erecting the boom and mast and also usually provides the power for the main hoist drum that vertically positions the load.
The rotary type cranes of a general type to which the present invention pertains utilizes counterbalancing means for the mast and boom. These cranes are of enormous size, the boom often extending hundreds of feet into the air and are used for moving extremely heavy equipment.
An example of a prior art device of this general character is shown in the U.S. Pat. No. 3,485,383 issued Dec. 23, 1963 and entitled "Axial Support for Cranes." Other examples of prior art rotary crane structures, but which do not employ a large supporting ring are shown in the U.S. Pat. Nos. 3,868,022 issued Feb. 25, 1975 and entitled "Self-Propelled Heavy Duty Mobile Crane" and which has been assigned to an assignee common with the present invention; U.S. Pat. No. 3,842,984 issued Oct. 22, 1974 and entitled "Crane Counter-Balancing Trailer Assembly," and U.S. Pat. No. 3,836,010 issued Sept. 17, 1974 entitled "Counter-Balancing Crane Structure."